Bug fix: IsWritable() returned true if the stream was readable.
Adds File IO methods to the File API

Adds new method `open` to File class. This method returns a stream object to a file given as an argument.

Also adds `close`, `read` and `write` methods to the stream object.

Adds O_CREAT and O_TRUNC flags when files are opened for writing in Filename class.
Merge pull request #95 from peelonet/stream-class

File IO Methods